Shortwave-infrared particles and cameracombine for breakthrough in vivo imaging

Compared to other wavelengths, shortwave-infrared (SWIR) light (1000 to
2000 nm) limits autofluorescence, light
absorption, and scattering, making biological tissue nearly transparent. For
these reasons, SWIR is well suited to in
vivo imaging, but a dearth of appropriate
imaging agents has gated its adoption for
life sciences imaging.

Now, a research group led by scientists at the Massachusetts Institute of
Technology (MIT; Cambridge, MA) has
developed narrow-range SWIR-emissive
indium arsenide (InAs)-based quantum
dots. 1 The researchers say that the particles provide a dramatically higher
emission quantum yield than other SWIR
probes, and can be easily modified for
various functional-imaging applications.

Their experiments with mice demonstrated important advantages of the dots:
deep tissue penetration, high spatial resolution, and multicolor optical imaging.
With the help of a state-of-the-art
camera, the particles also enabled high-speed image acquisition.

Camera-enabled

The researchers used the particles—witha commercially available camera featur-ing a thermoelectrically cooled indiumgallium arsenide (InGa As) focal-planearray (FPA) that is highly sensitive toSWIR light (NIRvana by PrincetonInstruments)—to quantify a number ofdynamics in awake, unrestrained animals.They measured metabolic turnover oflipoproteins in several organs simultane-ously and in real time, as well as heart-beat and breathing rates. Using thesereadings, they produced detailed, three-dimensional quantitative flow maps ofbrain vasculature (see figure).Researchers in the lab of MoungiBawendi, Lester Wolf Professor of Chem-istry, made the dots from semiconduc-tor materials. Tight control of the par-ticles’ size and composition enablesprecise matching of their light emissionto desired SWIR frequencies so that theycan be easily detected through the sur-rounding skin and muscle. The parti-cles are “orders of magnitude betterthan previous materials, and that allowsunprecedented detail in biologicalimaging,” according to research scientistOliver Bruns, who is first author on thepaper describing the work. The synthesisof these new particles was first describedby Daniel Franke and colleagues. 2

In a separate experiment, graphene
was used to demonstrate an even broader,
approximately 300 nm tuning range for a
chromium:zinc sulfide (Cr:ZnS) solid-state
laser, where the output is as short as 200
fs and wavelength selection is achieved
using a prism pair (see figure).

“Our preliminary results are quite prom-ising; they suggest that carbon nanoma-terials, including carbon nanotubes andgraphene, may be the key optical switch-ing materials enabling a new generationof broadly tunable mode-locked fiberlasers in the technologically important 2μm region,” Wang says. “It is quite likelythat these carbon-enabled systems willbe commercialized, considering the hugeadvantages in terms of footprint and costreduction.”— Gail Overton